IgA nephropathy (IgAN) is characterized by mesangial deposits of lgA1 with co-deposits of C3, and often also IgG or IgM or both. Circulating immune complexes with galactose-deficiency in the O-linked glycans in the hinge region of lgA1 play a major role in the pathogenesis of IgAN. It has been established that the content of galactose in the circulating lgA1 is lower than in healthy controls, that this results in the exposure of neoepitopes that are recognized by naturally occurring IgG or lgA1 antibodies resulting in the formation of immune complexes, and that the lgA1 eluted from nephrectomy specimens of patients with IgAN is galactose-deficient. It is not yet known whether the galactose deficiency affects all of these glycans or only a subset or single glycan. Based on our preliminary data, we have formulated the central hypothesis that the deficiency of galactose in the hinge region of lgA1 is pivotal for the pathogenesis of IgAN and that this O- glycosylation abnormality is a disease-specific epitope. We propose to test this hypothesis by defining the fine structure of the disease-specific epitope(s) and its location on the aberrantly-glycosylated lgA1;defining the characteristics of the lgA1-binding antibodies that promote their ability to form lgA1-complexes that can activate mesangial cells;and analyzing the binding of these immune complexes to mesangial cells. Based on these results, we will develop and critically evaluate the emerging serum biomarkers of IgAN and validate the findings in a separate cohort of well-characterized IgAN patients. The feasibility of these studies is enhanced greatly by our development of high through-put methods of analysis, EBV-immortalized cell lines that produce aberrantly glycosylated lgA1 or anti-glycan antibodies, state-of-the-art techniques for analysis of glycopeptides, and our extensive basic and clinical collaborations that permit testing of the hypothesis by experimental manipulation as well as clinical correlations. Relevance: IgAN is the most common primary glomerulonephritis and leads to the loss of renal function in 20-40% patients. Unfortunately, the diagnosis of IgAN still relies on renal biopsy due to the lack of noninvasive tests. Our studies will provide an improved understanding of the pathogenesis of IgAN and provide a basis for the development of a clinically applicable, high-throughput assay for noninvasive diagnosis of IgAN, and monitoring of the disease course or response to therapy. This would represent a major advance in the quality of care for IgAN patients.